The present invention relates to compounds, processes for their preparation, compositions containing them, to their use in the treatment of various disorders in particular allergic diseases and other inflammatory conditions for example allergic rhinitis and asthma, infectious diseases, cancer, and as vaccine adjuvants.
Vertebrates are constantly threatened by the invasion of microorganisms and have evolved mechanisms of immune defence to eliminate infective pathogens. In mammals, this immune system comprises two branches; innate immunity and acquired immunity. The first line of host defence is the innate immune system, which is mediated by macrophages and dendritic cells. Acquired immunity involves the elimination of pathogens at the late stages of infection and also enables the generation of immunological memory. Acquired immunity is highly specific, due to the vast repertoire of lymphocytes with antigen-specific receptors that have undergone gene rearrangement.
The innate immune response was originally thought to be non-specific, but is now known to be able to discriminate between self and a variety of pathogens. The innate immune system recognises microbes via a limited number of germline-encoded Pattern-Recognition Receptors (PRRs) which have a number of important characteristics.
Toll-like receptors (TLRs) are a family of ten Pattern Recognition Receptors described in man. TLRs are expressed predominantly by innate immune cells where their rôle is to monitor the environment for signs of infection and, on activation, mobilise defence mechanisms aimed at the elimination of invading pathogens. The early innate immune-responses triggered by TLRs limit the spread of infection, while the pro-inflammatory cytokines and chemokines that they induce lead to recruitment and activation of antigen presenting cells, B cells, and T cells. The TLRs can modulate the nature of the adaptive immune-responses to give appropriate protection via dendritic cell-activation and cytokine release (Akira S. et al, Nat. Immunol., 2001: 2, 675-680). The profile of the response seen from different TLR agonists depends on the cell type activated.
TLR7 is a member of the subgroup of TLRs (TLRs 3, 7, 8, and 9), localised in the endosomal compartment of cells which have become specialised to detect non-self nucleic acids. TLR7 plays a key rôle in anti-viral defence via the recognition of ssRNA (Diebold S. S. et al, Science, 2004: 303, 1529-1531; and Lund J. M. et al, PNAS, 2004: 101, 5598-5603). TLR7 has a restricted expression-profile in man and is expressed predominantly by B cells and plasmacytoid dendritic cells (pDC), and to a lesser extent by monocytes. Plasmacytoid DCs are a unique population of lymphoid-derived dendritic cells (0.2-0.8% of Peripheral Blood Mononuclear Cells (PBMCs)) which are the primary type interferon-producing cells secreting high levels of interferon-alpha (IFNα) and interferon-beta (IFNβ) in response to viral infections (Liu Y-J, Annu. Rev. Immunol., 2005: 23, 275-306).
Allergic diseases are associated with a Th2-biased immune-response to allergens. Th2 responses are associated with raised levels of IgE, which, via its effects on mast cells, promotes a hypersensitivity to allergens, resulting in the symptoms seen, for example, in allergic rhinitis. In healthy individuals the immune-response to allergens is more balanced with a mixed Th2/Th1 and regulatory T cell response. TLR7 ligands have been shown to reduce Th2 cytokine and enhance Th1 cytokine release in vitro and to ameliorate Th2-type inflammatory responses in allergic lung models in vivo (Fili L. et al, J. All. Clin. Immunol., 2006: 118, 511-517; Moisan J. et al, Am. J. Physiol. Lung Cell Mol. Physiol., 2006: 290, L987-995; Tao et al, Chin. Med. J., 2006: 119, 640-648). Thus TLR7 ligands have the potential to rebalance the immune-response seen in allergic individuals and lead to disease modification.
Central to the generation of an effective innate immune response in mammals are mechanisms which bring about the induction of interferons and other cytokines which act upon cells to induce a number of effects. These effects can include the activation of anti-infective gene expression, the activation of antigen presentation in cells to drive strong antigen-specific immunity and the promotion of phagocytosis in phagocytic cells.
Interferon was first described as a substance which could protect cells from viral infection (Isaacs & Lindemann, J. Virus Interference. Proc. R. Soc. Lon. Ser. B. Biol. Sci. 1957: 147, 258-267). In man, the type I interferons are a family of related proteins encoded by genes on chromosome 9 and encoding at least 13 isoforms of interferon alpha (IFNα) and one isoform of interferon beta (IFNβ). Recombinant IFNα was the first approved biological therapeutic and has become an important therapy in viral infections and in cancer. As well as direct antiviral activity on cells, interferons are known to be potent modulators of the immune response, acting on cells of the immune system.
As a first-line therapy for hepatitis C virus (HCV) disease, interferon combinations can be highly effective at reducing viral load and in some subjects in eliminating viral replication. However, many patients fail to show a sustained viral response and in these patients viral load is not controlled. Additionally, therapy with injected interferon may be associated with a number of unwanted adverse effects which are shown to affect compliance (Dudley T, et al, Gut., 2006: 55(9), 1362-3).
Administration of a small molecule compound which could stimulate the innate immune response, including the activation of type I interferons and other cytokines, could become an important strategy for the treatment or prevention of human diseases including viral infections. This type of immunomodulatory strategy has the potential to identify compounds which may be useful not only in infectious diseases but also in cancer (Krieg. Curr. Oncol. Rep., 2004: 6(2), 88-95), allergic diseases (Moisan J. et al, Am. J. Physiol. Lung Cell Mol. Physiol., 2006: 290, L987-995), other inflammatory conditions such as irritable bowel disease (Rakoff-Nahoum S., Cell., 2004, 23, 118(2): 229-41), and as vaccine adjuvants (Persing et al. Trends Microbiol. 2002: 10(10 Suppl), S32-7).
In animal models, imiquimod demonstrated adjuvant activities either topically (Adams S. et al, J. Immunol., 2008, 181:776-84; Johnston D. et al, Vaccine, 2006, 24:1958-65), or systemically (Fransen F. et al, Infect. Immun., 2007, 75:5939-46). Resiquimod and other related TLR7/8 agonists have also been shown to display adjuvant activity (Ma R. et al, Biochem. Biophys. Res. Commun., 2007, 361:537-42; Wille-Reece U. et al, Proc. Natl. Acad. Sci. USA, 2005, 102:15190-4; Wille-Reece U. et al, US2006045885 A1).
Mechanisms which lead to induction of type I interferons are only partly understood. One mechanism which can lead to the induction of interferon in many cell types is the recognition of double-stranded viral RNA by the RNA helicases RIG-I and MDA5. This mechanism is thought to be the primary mechanism by which interferons are induced by Sendai virus infection of cells.
Further mechanisms for the induction of interferons are via TLR-dependent signalling events. In man, plasmacytoid dendritic cells (pDCs) are professional interferon-producing cells, able to make large amounts of interferons in response to, for example, viral infection. These pDCs are shown to preferentially express TLR7 and TLR9 and stimulation of these receptors with viral RNA or DNA respectively can induce expression of interferon alpha.
Oligonucleotide agonists of TLR7 and TLR9, and small molecule purine-based agonists of TLR7 have been described which can induce interferon alpha from these cell types in animals and in man (Takeda K. et al, Annu. Rev. Immunol., 2003: 21, 335-76). TLR7 agonists include imidazoquinoline compounds such as imiquimod and resiquimod, oxoadenine analogues and also nucleoside analogues such as loxoribine and 7-thia-8-oxoguanosine which have long been known to induce interferon alpha. International Patent Application publication number WO 2007/034882 (Dainippon Sumitomo Pharma Co. Ltd./AstraZeneca Aktiebolag) discloses certain adenine compounds useful as medicine.
It remains unclear how small molecule purine-like compounds can induce type I interferons and other cytokines since the molecular targets of these known inducers have not been identified. However, an assay strategy has been developed to characterise small molecule inducers of human interferon IFNα (regardless of mechanism) which is based on stimulation of primary human donor cells with compounds, and is disclosed herein.